1. Field of the Invention
This invention relates in general to downhole tools and in particular to a pulser for measurement while drilling tools.
2. Background of the Invention
Measurements of drill string orientation must be made while drilling a well bore in order to successfully reach targeted production zones within the formation of the earth. These measurements must be transmitted to the surface in a timely manner even at vertical depths of over three thousand meters. Measurement while drilling (MWD) allows for the surface acquisition of downhole data during drilling, thereby reducing the need for costly and time consuming drill string tripping and logging/survey runs otherwise necessary to acquire downhole data. The MWD device is set behind the bit at the surface and remains there as the drill string bores downward and so must be self powered. It is therefore, important that the mechanism for communicating its orientation measurements or surveys to the surface must be power efficient.
In modern MWD systems, one common mechanism for communicating to the surface with is downhole pulsers. Pulsers generate pressure surges or pressure pulses in drilling fluid or mud which is flowing through a drill string. The MWD information is coded in a sequence of pulses so that it can be sensed or “read” at the surface. In one type of pulser, pulses are created by partially obstructing an orifice in the drill string through which the drilling mud is flowing with a signal poppet. The signal poppet is moved rapidly in and out of the orifice to create a pressure spike that can be detected at the surface. Some pulsers require many moving parts and require significant amounts of power which quickly deplete the energy reserves of battery-powered tools. An improved pulser is desirable.
Typical downhole measurement tools communicate to the surface by creating positive pressure pulses in the drilling fluid that is pumped downhole from the surface. These pressure pulses are achieved within a portion of the measurement tool called a mud pulser. The pulser has a piston which is driven into a nozzled flow restriction mounted within the drill string in order to create the pressure pulse. The power to drive the piston with great force into this restriction is provided by the hydraulic energy of the drilling fluid. The high velocity drilling fluid travels past a cavity or inner passage within the pulser, creating a vacuum which is used to hold the piston up and out of the orifice. This vacuum effect is referred to as the Bernoulli Effect. When a valve, positioned at the top of the vacuum cavity is opened, an equalization of the pressure within the cavity and the pressure outside the pulser drives the piston down into the orifice and creates the pressure pulse. When the valve is closed, a vacuum is again created and the piston is pulled out of the orifice. This valve is manipulated using autonomous power of the MWD device.
Mud pulsers of the style described above are used by several corporations. The variation between these similar devices mainly occurs in the size of poppet, sizes of the restriction nozzle within the pulser sub or orifice, the length of the pulser and the mechanism for manipulating the control valve. The mechanism largely dictates the electrical power required to create the pressure impulse and the other variations largely effect the size and shape of the pressure impulse.
The linear movement of the control valve has been accomplished in several ways. One prior assembly included a single solenoid which pulled the control valve open. In order to retain the valve in the open position, the solenoid had to remain energized. Returning of this valve to the closed position was accomplished with a return spring. This spring was directly opposing the solenoid and hence low levels of pull force were achieved and extra electrical power was expended to keep the valve open. Low levels of pull force cause operating limitations of the pulser as a communication device. A second design utilized two solenoids positioned back-to-back such that when activated, one pulled the valve open and the second pulled the valve closed when neither solenoid was energized. The compression force required to open or hold open the valve is lower than the force required to close the valve (which was provided by the closed solenoid) and so some pull force was gained and the electrical energy required to keep the valve open was reduced. With the solenoid designs, a large capacitive bank, continually charged, was required to store up the large amounts of electrical current expensed while energizing the solenoids. This capacitive bank would add greater than 3 decimeters to the length of the pulser.
A newer method of valve manipulating utilizes a direct current motor or direct current stepper motor to rotate a shaft connected to a mechanical device (such as a screw, worm screw or ball screw) which translates rotational movement into axial movement. With this method, greater force could be achieved in both the opening and closing of the valve while only using one manipulating device. In addition, no electrical current or spring is needed to hold the valve on either position. To prevent the motor from attempting to open or close the valve passed its safe operating extends a stop or position detection mechanism must be incorporated. The existing pulsers of this type employ a current monitoring technique. The electrical current drawn by the motor is electronically measured. The measurements are filtered either by digital or analog means and then the resulting signal is passed through an algorithm which detects when the motor has stalled or has begun to drive the valve passed its operating extents. Both of these conditions will create a great increase in the electrical current demanded by the motor. This position detection method has several undesirable limitations mainly due to the fact that valve interference is required for the detection. The motor shaft cannot be stopped instantaneously because of its angular momentum. If the shaft is turned at a high velocity in order to achieve the required rapid valve transition (such as in the solenoid designs) its momentum will carry it harder into the extents of valve travel even after the motor is no longer energized. The higher levels of motor current used to detect the position of the valve cause excess energy expense. Also, much greater electrical current is required to start the motor in the opposite direction because of the additional friction caused by the excess travel into the valves operating extents due to its momentum. This friction can be significant and will often result in the valve being “stuck” as the electrical power available diminishes. These continual collisions of the valve with its open and closed extents also cause significant mechanic wear.
When the amount of pulling force available for opening the valve is low, the hydraulic conditions needed to drive the piston into the pulser sub may hold the valve in the closed position and prevent the creation of a pressure pulse. To aid pulsers with low valve pull, varieties of piston sizes and pulser sub mariations have been created to maintain the manageable required valve pull force within a specific range of drill fluid flow rates. In order to operate over a broad range of flow rates, many combinations of pistons and pulser subs must be available. Some of these combinations may allow for valve movement but have negative or diminishing effects on the pressure pulse. A greater valve pull force allows for larger flow rate operating ranges without the necessity of carefully selecting the piston size and pulser sub restriction.